1. Field of the Invention
The invention is related to the field of visual displays and in particular to the field of colloidal light valve displays.
2. Prior Art
Visual displays based on the colloidal light valve principal are generally well known in the art. Colloidal light valves as first taught by Land in U.S. Pat. Nos. 1,955,923, issued Apr. 23, 1934 and 1,963,496, issued June 19, 1934, embody dichroic dipole particles suspended in a colloid of fluid medium in which the randomly oriented dichroic dipole particles can be caused to align under the influence of an electrostatic or magnetic fields. The suspended dipole particles may be dehydrated or dyed causing the randomly oriented colloid suspension to be opaque to incident light. For display purposes, it is generally preferred to treat the particles so that in the randomly oriented state, the particles absorb the incident light producing what is generally referred to as a dark field. When an electric field is impressed across the colloidal suspension, the particles align and the suspension becomes transparent.
The prior passive display art using colloidal valves teaches the use of specular or diffuse reflectors behind the colloidal suspension of dichroic particles to reflect the incident or ambient light passed through the activated and therefore transparent portion of the display. Specular reflective surfaces are undesirable in that the transmitted light is specularly reflected and observation of the reflected image is position sensitive with respect to the direction of the incident light. Further reflected images surrounding the observer are superimposed on the displayed image which detracts from the quality of the image. Diffuse reflectors have the disadvantage that only a smaller portion of the incident light re-emerges from the entrance window to produce the passive image. This is due to the polarizing effect of the aligned particles which permit only about one-half of the obliquely incident light to be transmitted to the diffuse reflector and only about one-half of the diffusely reflected light to be transmitted back and emerge from the front window. The brightness of the resulting image with diffuse reflectors is therefore not as bright as with a specular reflector especially when the viewing angle lies close to the angle of reflection of the light source.
Dobbins in U.S. Pat. No. 3,897,137, issued July 29, 1975, teaches one the solution of this problem. In his patent, Dobbins discloses a display in which a liquid crystal and a colloidal light valve are placed in tandem, i.e., one behind the other. When activated, the transmissive portion of the liquid crystal valve is turbid (translucent), therefore, the light transmitted through the liquid crystal cell and received by the colloidal valve is diffused. The colloidal valve has a specular reflective surface behind the colloidal suspension and therefore one-half of the received light emerges back out the entrance window. The light emerging from the colloidal light valve is again diffusely transmitted by the liquid crystal valve producing a diffuse image having a higher contrast than a colloidal display having only a diffuse reflective surface. The disclosed tandem liquid crystal cell and colloidal valve eliminates the problems discussed with respect to colloidal displays having only a specular reflective surface.
Disclosed herein is a colloidal display having a high contrast diffuse image comparable to that disclosed by Dobbins without requiring an additional liquid crystal display.